Proteins can be reduced to their constituent amino acids. All of the 20 amino acids found in proteins have a carboxyl group and an amino group bonded to the same carbon atom (the a-carbon). For all the standard amino acids except one, glycine, the a-carbon is asymmetric, bonded to four different substituent groups: a carboxyl group, an amino group, an R group and a hydrogen atom. The a-carbon is thus a chiral centre.

They differ from each other in their side chains, or R groups, which vary in structure, size and electric charge, and influence the solubility of amino acids in water. Amino acids in aqueous solution are ionized and can act as acids or bases, an important point in understanding the physical and biological properties of proteins. Those a-amino acids having a single amino group and a single carboxyl group crystallize from neutral aqueous solutions as fully ionized species known as zwitterions (German for “hybrid ions”), each having both a positive and a negative charge.

There are five main classes of amino acids, those whose R groups are: nonpolar and aliphatic, aromatic (generally non polar); polar but uncharged; negatively charged and positively charged. Within each class there are graduations of polarity, size and shape of the R groups.

Another piece of information derived from the polar properties of amino acids is the titration curve, where each one of them have a characteristic shape.